The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air hearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. Accordingly, an important and ongoing goal involves increasing the amount of information able to be stored in the limited area and volume of HDDs. Increasing the areal recording density of HDDs provides one technical approach to achieve this goal.
A real density, e.g., as measured in bits per square inch, may be defined as the product of the track density (the tracks per inch radially on the magnetic medium, such as a disk) and the linear density (the bits per inch along each track). For a disk, the bits are written closely-spaced to form circular tracks on the disk surface, where each of the bits may comprise an ensemble of magnetic grains.
An important factor relevant to track density is the magnetic write width (MWW). The magnetic write width determines the track width of a magnetic bit recorded by the write/main pole of the write head. Thus, the smaller the magnetic core width, the greater the number of tracks of data that can be written to the media. Stated another way, high track density is associated with a narrow magnetic write width. However, writing narrower tracks generally involves narrowing the width of the poles on the read/write head, which ultimately reduces the strength of the head's write field. Unfortunately, weaker head write fields may result in a degradation of the writeability (e.g., the ability to switch the magnetization of the recording bits).
Perpendicular magnetic recording media typically have a layered structure in which the following layers may be stacked in succession on a substrate: a soft magnetic underlayer (SUL), a nonmagnetic intermediate layer, a recording layer, a carbon overcoat layer, and a lubricant layer. Most perpendicular recording media have a SUL with an antiferromagnetic coupling (AFC) structure in which two layers having amorphous magnetic alloys therein are antiferromagnetically coupled through a very thin coupling layer including a Ru or Ru alloy. A SUL with an AFC structure is known to provide lower noise than a SUL having no AFC structure. One of the most effective ways to improve writeability in the perpendicular magnetic recording media is to decrease the saturation magnetic flux density (Bs) of the SUL. For instance, a SUL, particularly a SUL with an AFC structure, that has a lower Bs also has a higher magnetic permeability. The writeability of a perpendicular magnetic media may therefore be improved by decreasing the Bs of the SUL, particularly a SUL with an AFC structure. Accordingly, a SUL with low Bs may be preferable for perpendicular recording media that is used in conjunction with a narrow track magnetic recording head (i.e., a recording head associated with a narrow magnetic write width and a weak write field). In current practice, the majority of perpendicular recording media have a SUL with a Bs below 1 Tesla.
However, there is currently a deficiency in SULs that have optimum low magnetic flux densities at higher temperatures. In general, the Bs associated with a SUL decreases at a higher rate with increasing temperature, which may be particularly problematic for SULs that have a low initial Bs at ambient temperatures. In other words, a SUL with a Bs that is optimum at room temperature may not be optimum at high temperature. If the Bs of a SUL becomes too low with increasing temperature, the writeability may be degraded because the magnetic moment in the SUL at high temperature will be saturated by the head field. Indeed, some of the recent perpendicular recording media exhibit poor writeability at high temperature. Accordingly, this deficiency in SULs that have optimum low magnetic flux densities at higher temperatures significantly impacts which media may be used in high temperature conditions, such as vehicle storage systems. As such, to achieve high areal density using a low-Bs SUL there is currently a need in the art to suppress the decreasing rate of magnetic flux density (Bs) with increasing temperature.